As used herein, the term tunneling refers to a quantum mechanical phenomenon where a sub-atomic particle, e.g., an electron, tunnels through a barrier, e.g., a classically forbidden energy state, that it classically could not surmount. A simple tunneling barrier can be created by separating two conductors with a very thin insulator. This can be represented in a semiconductor device by an electron tunneling from a semiconductor material through a dielectric material, which represents the energy barrier, to a conductive material on the other side. Even if the energy barrier presented by the dielectric material is higher than the electron energy, there is quantum-mechanically a finite probability of this transition through the dielectric layer.
Thus, a tunneling layer provides a physical barrier to particles, but not an electrical barrier to sub-atomic electrons and electrical current. Such a tunneling layer will not increase resistance in an electrical device because electrons do not need to overcome an energy barrier to pass the tunneling layer. However, such a tunneling layer can be a physical barrier to the movement of particles larger than electrons, e.g., atoms or ions.
In any event, a photovoltaic (PV) device is a device that can convert photo-radiation into electrical current. A typical PV device includes two conductive electrodes sandwiching a series of semiconductor layers, which provide a junction at which photoconversion occurs. During operation, photons pass through the PV device layers and are absorbed at or near the junction. This produces photo-generated electron-hole pairs, the movement of which, promoted by a built-in electric field, produces electric current that can be output from the device. A PV device can be a PV cell, PV module, etc. A PV module is made of a plurality of connected PV cells.
PV modules can be formed on an optically transparent substrate of any suitable, transparent substrate material. Suitable materials include, for example, glass, such as soda-lime glass or float glass, etc., and polymer (sheet or plates). A first of the two conductive electrodes is provided over the transparent substrate. The first conductive electrode can be a transparent conductive oxide (TCO) layer (e.g., indium tin oxide). The TCO layer can also be associated with a barrier layer, which can be conductive, between it and the transparent substrate and a conductive oxide buffer layer over the TCO layer, which together provide a conductive TCO stack that functions as the first conductive electrode. Over the first conductive electrode (e.g., over the buffer layer if provided) a semiconductor layer can be provided. The semiconductor layer can be a bi-layer that includes a semiconductor window layer (e.g., cadmium sulfide) and a semiconductor absorber layer (e.g., cadmium telluride). Over the semiconductor layer, the second of the two conductive electrodes can be a back contact layer. A back cover can be provided over the back contact layer to provide support for the PV module. An interlayer can be provided between the back contact layer and the back cover and over the sides of the other layers of the PV module to seal the PV module from the environment.
Efficiency, stability, and reliability in PV module performance are always goals in PV module manufacturing. These depend at least in part on materials used in the component layers of the PV module, the position of each layer relative to each other, and the thickness of each layer. For example, one way a PV module can be made more efficient is by thinning the window layer because the materials commonly used for this component (e.g., CdS) are fairly light absorbent, particularly to blue wavelength light. A thinner window layer can allow more light to be transmitted to the absorber layer, thereby allowing more photoconversion of electricity. However, in making the window thinner problems can occur, such as having missing portions of the window layer material where the underlying conductive TCO stack may come into direct electrical contact with the absorber layer. This electrical contact between the absorber layer and TCO stack could cause the PV module to malfunction. For example, electrical shunting (a conductive path through an otherwise non-conducting layer) or shorting (unwanted direct electrical contact between materials) between the absorber and conductive TCO stack could be exhibited, which can make the PV device unstable.
Furthermore, during field operation of a PV module, it is possible for the materials of some layers within the module to migrate to other layers within the PV module under the influence of the electrical current caused by photoconversion. For example, Mg2+, Na+, and/or Ca2+ ions from the glass substrate of the PV module could migrate to the absorber layer, which could significantly degrade the performance of the PV module by changing the electrical characteristics of the absorber layer or making it sensitive to moisture.
A PV module structure which mitigates against such shorting/shunting and particle migration problems is desired.